Nydus Image Service v2.1.0-alpha.6

What's Changed

• storage: remove localfs based prefetch by @jiangliu in #628
• storage: unify method naming style for ChunkInfo by @changweige in #632
• remove dead code nydus-cached by @changweige in #635
• builder: fix broken symlink in unpack by @imeoer in #636
• Improve prefetch performance further by @jiangliu in #633
• ci: remove nydus-cached binary build by @imeoer in #637

Full Changelog: v2.1.0-alpha.5...v2.1.0-alpha.6